1. Field of the Invention
The invention relates to a device for determining the fill level of a filling substance in a container having a signal generating unit which generates high-frequency signals, having a transmitter/receiver unit, which transmits the signals via an antenna and receives the signals reflected from the surface of the filling substance, having a coaxial line with an inner conductor and an outer conductor for carrying the signals, and having an evaluation unit, which from the transit time of the signals ascertains the fill level in the container; the antenna has a waveguide, defined by a back wall, and a send line is provided on the back wall of the waveguide, which send line extends essentially inside the waveguide, and a first end section of the send line is connected to the inner conductor of the coaxial line.
2. Related Art
Transit time methods utilize the physical principle according to which the transit distance is equal to the product of the transit time and the rate of propagation. In the case of fill level measurement, the transit distance equals twice the spacing between the antenna and the surface of the filling substance. The useful echo signal, that is, the signal reflected from the surface of the filling substance, and its transit time are determined from the so-called echo function, or digitized envelope curve, and the envelope curve represents the amplitudes of the echo signals as a function of the spacing between the antenna and the surface of the filling substance. The fill level itself is then obtained from the difference between the known spacing between the antenna and the bottom of the container and the spacing, determined by the measurement, between the surface of the filling substance and the antenna. All the known methods that make it possible to measure relatively short distances by means of reflected microwaves can be used. The best-known examples are pulse radar and frequency modulation continuous wave radar (FMCW radar).
In pulse radar, short microwave pulses are transmitted periodically. In the FMCW method, a continuous microwave is transmitted, which is periodically frequency-modulated linearly, for instance in accordance with a sawtooth function. The frequency of the received echo signal therefore, compared to the frequency that the transmitted signal has at the instant of reception, has a frequency difference, which depends on the transit time of the echo signal. The frequency difference between the transmitted signal and the received signal, which can be obtained by mixing the two signals and evaluating the Fourier spectrum of the mixed signal, is thus equivalent to the spacing between the reflecting surface and the antenna. Moreover, the amplitudes of the spectral lines of the frequency spectrum, obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum in this case therefore represents the echo function.
Electromagnetic waves propagate in coaxial lines without dispersion by the transversal-electromagnetic mode (TEM mode). This mode is therefore especially well suited for transporting wave packets or electromagnetic waves that have a certain frequency bandwidth. Fed-in wave packets undergo practically no propagation in that case; in linearly frequency-modulated microwaves as well, a deviation in linearity is largely avoided.
For oriented transmission of electromagnetic waves by means of an antenna, modes are preferably employed whose broadcast characteristic has a pronounced forward lobe. The transverse electric 11 mode (TE11 mode), which is capable of propagation in round waveguides, has this property. As a function of the dimensions of the antenna used as the waveguide, a frequency range exists within which the TE11 mode is the only mode capable of propagation. Above this frequency range, higher modes, such as the TM01 mode, are also capable of propagation but are less well suited to the oriented transmission of microwaves.
From German Utility Model DE-G 93 12 251.9, it has become known to incorporate the sending lobe laterally into the antenna embodied as a round waveguide. A disadvantage of such an arrangement is that the laterally positioned sending lobe generally requires an additional housing, for protecting the coaxial line connected to the sending lobe. This increases the diameter, compared to an arrangement in which the inputting of the microwaves takes place through a back wall of the antenna. Another disadvantage of this prior art is that because of the asymmetry of the arrangement, not only the TE11 mode but higher modes as well are excited. Higher modes, however, have a different broadcast characteristic and are therefore less well suited to oriented broadcasting.
From European Patent Disclosure EP 0 821 431 A2, a device has become known which is capable of generating a mode whose broadcast characteristic has a pronounced forward lobe. Moreover, this device can be used over a wide frequency range. To that end, the signals are input from the back side of the round waveguide acting as an antenna. The inputting itself is done over a trapezoidal send line, which is disposed on the back wall of the antenna and extends essentially within the interior of the antenna. One end of the send line is connected to the inner conductor of the coaxial line carrying the signals; the other end of the send line is put into electrical contact with the back wall of the antenna.
It is the object of the invention to provide a device for determining the fill level of a filling substance in a container that is distinguished by a simple structure.
This object is attained in that a second end section of the send line is disposed freely and essentially parallel to the back wall of the waveguide; and that the spacing between the second end section of the send line and the back wall of the waveguide is essentially xcex/8, where xcex is the wavelength of the high-frequency signals carried in the waveguide. The symbol xcex represents the wavelength of the high-frequency signals carried in the waveguide at a frequency to be transmitted. This frequency is for instance the mean frequency in the frequency spectrum of a high-frequency pulse to be transmitted, or the mean frequency of a linearly frequency-modulated FMCW transmission signal.
Because of the axial inputting, the device of the invention is distinguished by a slender design. The inputting does notxe2x80x94as in the case of lateral inputtingxe2x80x94exceed the dimensions of the round waveguide. It can therefore be mounted without difficulty even on containers with narrow openings. Moreover, because of the straight inputting, neither angled plugs nor line angles, which would adversely affect the propagation of the high-frequency signals, are necessary. In addition, because of the slight spacing of xcex/8 between the second end section of the send line and the back wall, the antenna can have shorter dimensions than the antennas known until now. In them, the corresponding spacing has always been given as xcex/4.
In an advantageous embodiment of the device of the invention, it is provided that the coaxial line is flush at the front with the back wall of the waveguide. As a result, the high mechanical effort and expense necessary in the case of German Patent Disclosure DE 195 45 493 A1 for passing the coaxial line through the back wall into the waveguide of the antenna is avoided.
An advantageous embodiment of the device of the invention provides that the first end section and the second end section of the send line are disposed essentially perpendicular to one another. The advantage of this embodiment is self-evident: The send line can be fabricated quite simply by appropriate bending. Moreover, the disposition of a sending lobe on the free end, as provided for instance in DE 195 45 493 A1, is superfluous in conjunction with the device of the invention.
In a preferred refinement of the device of the invention, the send line is disposed on a printed circuit board secured in the antenna. This makes producing the device of the invention substantially easier.
Moreover, an advantageous embodiment of the device of the invention provides a dielectric in the waveguide of the antenna, which fills the waveguide at least in the environment of the send line. The dielectric is for instance PTFE (polytetrafluoroethylene).